In U.S. Pat. Nos. 3,820,472; 3,967,561 and 4,027,597, for example, linear induction motors are described which comprise a stator or primary member, complete with windings and pole pieces, and a secondary part, such as an armature or reaction rail of high conductivity in which eddy currents are generated to electromagnetically react with the field of the primary part and generate a force tending to displace one of the parts relative to the other.
When such systems are provided in vehicles, e.g. air-cushion or magnetically-suspended vehicles, they constitute a particularly efficient and compact propulsion source for driving a vehicle along a track. The vehicle may carry the primary part or stator and the track may be formed with the rail and can be constituted as a concrete structure along which the rail extends.
For such asynchronous linear induction motors (LIM) the rail is normally constituted of a material having high electrical conductivity, preferably aluminum.
Because of the different coefficients of thermal expansion of the reinforced concrete track and the metal reaction or armature rail attached thereto, the LIM rail cannot be continuous, i.e. the end of one sectional length of the rail cannot be contiguously and rigidly connected to the opposite end of the next rail section or length.
Between the proximal ends of successive rail sections or lengths, therefore, it is necessary to provide expansion gaps to permit the yielding of one LIM rail section relative to the other.
However, at these expansion gaps, the current-flow patterns induced in the LIM rail by the primary part or stator of the LIM are disturbed or distorted which results in a discontinuity in the vehicle-propulsion force at such gaps.
In addition, when the vehicle is traveling at relatively low speeds, i.e. when there is a slow relative speed between the primary and secondary parts of the linear induction motor, especially during start-up, a temperature increase is manifested at the expansion gap because of an increased concentration at the boundary edges thereof of the induced current. This temperature increase can be sufficient to cause fusion or melting of the rail materials at this gap and hence disruption of the operation of the system.
The problem has been recognized heretofore and there have been various attempts at solution. For example, it has been proposed to provide a galvanic connection between the two rail ends at an expansion gap via a flexible electrical conductor. While this system affords a better distribution of the electrical current than is the case where the gap is not spanned by a conductor, it is not entirely satisfactory because the flexible connector is of relatively high cost, can be mounted only at considerable expense, tends to increase the spacing which must be provided between the rail section faces and the juxtaposed pole faces of the stator, and is susceptible to deformation by the forces corresponding to the induced currents. In fact, the flexible connectors are frequently torn away by such forces and interfere with proper operation of the linear induction motor.
Apart from this, the system does not adequately provide for maintenance of the propulsion force across the expansion gap.